1. Field of the Invention
The present invention pertains to an improved piston ring for use in reciprocating pumps, compressors, expansion engines and the like. In particular, the present invention relates to a piston ring for use with a device for producing a cryogenic refrigeration such as shown in U.S. Pat. No. 3,620,029. Cryogenic refrigeration generally relates to temperatures at minus 150.degree. F. down to minus 450.degree. F.
2. Description of the Prior Art
U.S. Pat. Nos. 3,373,999 and 3,540,746 disclose piston rings for use with refrigeration devices such as compressors for producing refrigeration at cryogenic temperatures. The piston rings disclosed in the two patents are of the dry lubricated type since at temperatures of minus 150.degree. to minus 450.degree. F. conventional lubrication oils would freeze.
Although rings of the type shown in the aforementioned patents have been found to be satisfactory for compressors, they have been found to be less than effective for a refrigeration device shown in U.S. Pat. No. 3,620,029. When applied to a displacer-expander type refrigerator as shown in the last-mentioned patent, the patented piston rings, while exhibiting better leakage characteristics than conventional rings, tend to show some minor leakage between the piston and the ring, thereby, decreasing the overall efficiency of the refrigeration device. This is especially true for a device shown in FIG. 3 of the last-named patent, wherein there is a two-stage displacer for producing two levels of refrigeration.